1. Field of the Invention
The present invention relates to an analysis method of finding a mixing ratio with respect to a nitride semiconductor used for an electronic device, a light emitting device, and a high frequency device, particularly with respect to a ternary mixed-crystal nitride semiconductor layer, and a method of manufacturing a nitride semiconductor substrate using the analysis method.
2. Description of the Related Art
In recent years, a nitride semiconductor substrate has been developed in which a nitride semiconductor layer of gallium nitride (GaN) suitable for a high withstand-voltage power device is deposited and formed on a substrate.
As an example of such, there may be mentioned a high electron mobility transistor (HEMT) with a structure where GaN layers as electron transit layers and AlxGa1-xN single crystal layers (0≦x≦1) as electron supply layers are stacked one by one on a Si substrate through a buffer layer.
In a HEMT substrate, it is especially important to precisely control x in the AlxGa1-xN single crystal layer (0≦x≦1) of the electron supply layer, i.e., a mixing ratio. As a method of measuring the mixing ratio of a mixed-crystal nitride semiconductor, the Rutherford backscattering method, the photo-luminescence method, etc. are known.
As an example of the method of measuring the mixing ratio by the photo-luminescence method, Japanese Patent Application Publication No. H1-94247 (Patent Literature 1) discloses a method in which excitation light by dye laser having optical energy higher than that of the band gap of the mixed crystal is applied to a sample surface to obtain a photo-luminescence spectrum generated from a sample, a penetration depth P of the excitation light inside a thin film sample is changed by changing an oscillation wavelength of the dye laser to obtain a photo-luminescence spectrum as a function of the penetration depth P, a band gap Eg is obtained as a function of a depth D from a combination of the photo-luminescence spectra, and the mixing ratio x by depth D is found from the band gap Eg.
Further, Japanese Patent Application Publication No. H7-63668 (Patent Literature 2) discloses that the sample surface irradiated with intensity-modulated excitation light is irradiated with probe light, and a reflection spectrum synchronized with the modulation is detected from the reflection light, so that a compound composition is found from energy at an absorption edge of the sample in which the energy is found from this reflection spectrum.
However, although the Rutherford backscattering method is excellent in measurement accuracy, an apparatus therefor is large in size and costly, and the measurement is carried out under vacuum, so that it requires time and effort for measurement per one sample. Therefore, it cannot be said that it is suitable for a simple and non-destructive measuring method to be used by incorporating itself into the manufacture process of the nitride semiconductor substrate especially.
Further, although the method by way of the photo-luminescence method described in Patent Literature 1 allows simple and non-destructive measurement, there is a possibility that it may be influenced by crystallinity, dislocation, etc., of a semiconductor layer of a measuring object, and it is hard to say that the stability and reliability of the measurement are fully secured. Furthermore, in the case where a film is formed on a surface of the semiconductor layer of the measuring object, the measurement is difficult because of the luminescence disturbance due to a quantum well structure.
Still further, Patent Literature 2 says that the technology described therein allows the composition of the compound semiconductor to be obtained easily and precisely compared with a fluorescent X-ray analysis. However, the modulation intensity of the reflection light also has a possibility that it may be influenced by crystallinity, dislocation, etc., of the semiconductor layer of the measuring object, and this technology does not fully secure the stability and reliability of the measurement.